The present invention is directed to lyocell fibers having novel characteristics and to the method for their preparation. It is also directed to yarns produced from the fibers, and to woven and nonwoven fabrics containing the fibers. In particular, the method involves first dissolving cellulose in an amine oxide to form a dope. Latent fibers are then produced either by extrusion of the dope through small apertures into an air stream which draws the latent filaments of cellulose solution or by centrifugally expelling the dope through small apertures. The fibers are then formed by regenerating the spun latent fibers in a liquid nonsolvent. Either process is amenable to the production of self bonded nonwoven fabrics.
For over a century strong fibers of regenerated cellulose have been produced by the viscose and cuprammonium processes. The latter process was first patented in 1890 and the viscose process two years later. In the viscose process cellulose is first steeped in a mercerizing strength caustic soda solution to form an alkali cellulose. This is reacted with carbon disulfide to form cellulose xanthate which is then dissolved in dilute caustic soda solution. After filtration and deaeration, the xanthate solution is extruded from submerged spinnerets into a regenerating bath of sulfuric acid, sodium sulfate, zinc sulfate, and glucose to form continuous filaments. The resulting so-called viscose rayon is presently used in textiles and was formerly widely used as reinforcing in rubber articles such as tires and drive belts.
Cellulose is also soluble in a solution of ammoniacal copper oxide. This property formed the basis for production of cuprammonium rayon. The cellulose solution is forced through submerged spinnerets into a solution of 5% caustic soda or dilute sulfuric acid to form the fibers. After decoppering and washing the resulting fibers have great wet strength. Cuprammonium rayon is available in fibers of very low deniers and is used almost exclusively in textiles.
More recently other cellulose solvents have been explored. One such solvent is based on a solution of nitrogen tetroxide in dimethyl formamide. While much research was done, no commercial process has resulted for forming regenerated cellulose fibers using this solvent.
The usefulness of tertiary amine N-oxides as cellulose solvents has been known for a considerable time. Graenacher, in U.S. Pat. No. 2,179,181, discloses a group of amine oxide materials suitable as solvents. However, the inventor was only able to form solutions with low concentrations of cellulose and solvent recovery presented a major problem. Johnson, in U.S. Pat. No. 3,447,939, describes the use of anhydrous N-methylmorpholine-N-oxide (NMMO) and other amine N-oxides as solvents for cellulose and many other natural and synthetic polymers. Again the solutions were of relatively low solids content. In his later U.S. Pat. No. 3,508,941, Johnson proposed mixing in solution a wide variety of natural and synthetic polymers to form intimate blends with cellulose. A nonsolvent for cellulose such as dimethylsulfoxide was added to reduce dope viscosity. The polymer solution was spun directly into cold methanol but the resulting filaments were of relatively low strength.
However, beginning in 1979 a series of patents were issued to preparation of regenerated cellulose fibers using various amine oxides as solvents. In particular, N-methylmorpholine-N-oxide with about 12% water present proved to be a particularly useful solvent. The cellulose was dissolved in the solvent under heated conditions, usually in the range of 90xc2x0 C. to 130xc2x0 C., and extruded from a multiplicity of fine apertured spinnerets into air. The filaments of cellulose dope are continuously mechanically drawn in air by a factor in the range of about three to ten times to cause molecular orientation. They are then led into a nonsolvent, usually water, to regenerate the cellulose. Other regeneration solvents, such as lower aliphatic alcohols, have also been suggested. Examples of the process are detailed in McCorsley and McCorsley et al. U.S. Pat. Nos. 4,142,913; 4,144,080; 4,211,574; 4,246,221, and 4,416,698 and others. Jurkovic et al., in U.S. Pat. No. 5,252,284 and Michels et al., in U.S. Pat. No. 5,417,909 deal especially with the geometry of extrusion nozzles for spinning cellulose dissolved in NMMO. Brandner et al., in U.S. Pat. No. 4,426,228, is exemplary of a considerable number of patents that disclose the use of various compounds to act as stabilizers in order to prevent cellulose and/or solvent degradation in the heated NMMO solution. Franks et al., in U.S. Pat. Nos. 4,145,532 and 4,196,282, deal with the difficulties of dissolving cellulose in amine oxide solvents and of achieving higher concentrations of cellulose.
Cellulose textile fibers spun from NMMO solution are referred to as lyocell fibers. Lyocell is an accepted generic term for a fiber composed of cellulose precipitated from an organic solution in which no substitution of hydroxyl groups takes place and no chemical intermediates are formed. One lyocell product produced by Courtaulds, Ltd. is presently commercially available as Tencel(copyright) fiber. These fibers are available in 0.9-2.7 denier weights and heavier. Denier is the weight in grams of 9000 meters of a fiber. Because of their fineness, yarns made from them produce fabrics having extremely pleasing hands.
One limitation of the lyocell fibers made presently is a function of their geometry. They are continuously formed and typically have quite uniform, generally circular or oval cross sections, lack crimp as spun, and have relatively smooth, glossy surfaces. This makes them less than ideal as staple fibers since it is difficult to achieve uniform separation in the carding process and can result in non-uniform blending and uneven yarn. In part to correct the problem of straight fibers, man made staple fibers are almost always crimped in a secondary process prior to being chopped to length. Examples of crimping can be seen in U.S. Pat. Nos. 5,591,388 or 5,601,765 to Sellars et al. where the fiber tow is compressed in a stuffer box and heated with dry steam. It might also be noted that fibers having a continuously uniform cross section and glossy surface produce yarns tending to have a xe2x80x9cplasticxe2x80x9d appearance. Yarns made from thermoplastic polymers frequently must have delustering agents, such as titanium dioxide, added prior to spinning. Wilkes et al., in U.S. Pat. No. 5,458,835, teach the manufacture of viscose rayon fibers having cruciform and other cross sections. U.S. Pat. No. 5,417,909 to Michels et al. discloses the use of profiled spinnerets to produce lyocell fibers having non-circular cross sections but the present inventors are not aware of any commercial use of this method.
Two widely recognized problems of lyocell fabrics are caused by fibrillation of the fibers under conditions of wet abrasion, such as might result during laundering. Fibrillation tends to cause xe2x80x9cpillingxe2x80x9d; i.e., entanglement of fibrils into small relatively dense balls. It is also responsible for a xe2x80x9cfrostedxe2x80x9d appearance in dyed fabrics. Fibrillation is believed to be caused by the high orientation and apparent poor lateral cohesion within the fibers. There is an extensive technical and patent literature discussing the problem and proposed solutions. As examples, reference might be made to papers by Mortimer, S. A. and A. A. Pxc3xa9guy, Journal of Applied Polymer Science, 60:305-316 (1996) and Nicholai M., A. Nechwatal, and K. P. Mieck, Textile Research Journal, 66(9):575-580 (1996). The first authors attempt to deal with the problem by modifying the temperature, relative humidity, gap length, and residence time in the air gap zone between extrusion and dissolution. Nicholai et al. suggest crosslinking the fiber but note that xe2x80x9c. . . at the moment, technical implementation [of the various proposals] does not seem to be likelyxe2x80x9d. A sampling of related United States Patents might include those to Taylor, U.S. Pat. Nos. 5,403,530, 5,520,869, 5,580,354, and 5,580,356; Urben, U.S. Pat. No. 5,562,739; and Weigel et al. U.S. Pat. No. 5,618,483. These patents mostly relate to treatment of the fibers with reactive materials to induce surface modification or crosslinking. Enzymatic treatment of yarns or fabrics is currently the preferred way of reducing problems caused by fibrillation. However, all of the treatments noted have disadvantages and increase the cost. A fiber that was resistant to fibrillation would be a significant advantage.
Kaneko et al. in U.S. Pat. No. 3,833,438 teach preparation of self bonded cellulose nonwoven materials made by the cuprammonium rayon process. Self bonded lyocell nonwoven webs have not been described to the best of the present inventors"" knowledge.
Low denier fibers from synthetic polymers have been produced by a number of extrusion processes. Three of these are relevant to the present invention. One is generally termed xe2x80x9cmelt blowingxe2x80x9d. The molten polymers are extruded through a series of small diameter orifices into an air stream flowing generally parallel to the extruded fibers. This draws or stretches the fibers as they cool. The stretching serves two purposes. It causes some degree of longitudinal molecular orientation and reduces the ultimate fiber diameter. A somewhat similar process is called xe2x80x9cspunbondingxe2x80x9d where the fiber is extruded into a tube and stretched by an air flow through the tube caused by a vacuum at the distal end. In general, spunbonded fibers are continuous while melt blown fibers are more usually in discrete shorter lengths. The other process, termed xe2x80x9ccentrifugal spinningxe2x80x9d, differs in that the molten polymer is expelled from apertures in the sidewalls; of a rapidly spinning drum. The fibers are drawn somewhat by air resistance as the drum rotates. However, there is not usually a strong air stream present as in meltblowing. All three processes may be used to make nonwoven fabric materials. There is an extensive patent and general technical literature on the processes since they have been commercially important for many years. Exemplary patents to meltblowing are Weber et al., U.S. Pat. No. 3,959,421, and Milligan et al., U.S. Pat. No. 5,075,068. The Weber et al. patent uses a water spray in the gas stream to rapidly cool the fibers. A somewhat related process is described in PCT Publication WO 91/18682 which is directed to a method for coating paper by modified meltblowing. Coating materials suggested are aqueous liquids such as xe2x80x9can aqueous solution of starch, carboxy-methylcellulose, polyvinyl alcohol, latex, a suspension of bacterial cellulose, or any aqueous material, solution or emulsionxe2x80x9d. However, this process actually atomizes the extruded material rather than forms it into latent fibers. Zikeli et al., in U.S. Pat. Nos. 5,589,125 and 5,607,639, direct a stream of air transversely across strands of extruded lyocell dope as they leave the spinnerets. This air stream serves only to cool and does not act to stretch the filaments.
Centrifugal spinning is exemplified in U.S. Pat. Nos. 5,242,633 and 5,326,241 to Rook et al. Okada et al., in U.S. Pat. No. 4,440,700 describe a centrifugal spinning process for thermoplastic materials. As the material is ejected the fibers are caught on an annular form surrounding the spinning head and moved downward by a curtain of flowing cooling liquid. Included among the list of polymers suited to the process are polyvinyl alcohol and polyacrylonitrile. In the case of these two materials they are spun xe2x80x9cwetxe2x80x9d; i.e., in solution, and a xe2x80x9ccoagulation bathxe2x80x9d is substituted for the curtain of cooling liquid.
With the exception of the Kaneko et al. patent noted above, processes analogous to melt blowing, spunbonding and centrifugal spinning have never been used with cellulosic materials since cellulose itself is basically infusible.
Extremely fine fibers, termed xe2x80x9cmicrodenier fibersxe2x80x9d generally are regarded as those having a denier of 1.0 or less. Meltblown fibers produced from various synthetic polymers, such as polypropylene, nylons, or polyesters are available with diameters as low as 0.4 xcexcm (approximately 0.001 denier). However, the strength or xe2x80x9ctenacityxe2x80x9d of most of these fibers tends to be low and their generally poor water absorbency is a negative factor when they are used in fabrics for clothing. Microdenier cellulose fibers, as low as 0.5 denier, have been produced before the present only by the viscose process.
The present process produces a new lyocell fiber that overcomes many of the limitations of the fibers produced from synthetic polymers, rayons, and the presently available lyocell fibers. It allows formation of fibers of low denier and with a distribution of deniers. At the same time, the surface of each fiber tends to be pebbled, as seen at high magnification, and the fibers have a cross section of varying shape and diameter along their length, have significant natural crimp, and are resistant to fibrillation under conditions of wet abrasion. All of these are desirable characteristics that are found in most natural fibers but are missing in lyocell fibers produced commercially to the present.
The present invention is directed to a process for production of regenerated cellulose fibers and webs and to the fibers and webs so produced. The terms xe2x80x9ccellulosexe2x80x9d and xe2x80x9cregenerated cellulosexe2x80x9d as used here should be construed sufficiently broadly to encompass blends of cellulose with other natural and synthetic polymers, mutually soluble in a spinning solvent, in which cellulose is the principal component by weight. In particular it is directed to low denier fibers produced from cellulose solutions in amine N-oxides by processes analogous to melt blowing or centrifugal spinning. Where the terms xe2x80x9cmelt blowingxe2x80x9d, xe2x80x9cspunbondingxe2x80x9d, and xe2x80x9ccentrifugal spinningxe2x80x9d are used it will be understood that these refer to processes that are similar or analogous to the processes used for production of thermoplastic fibers, even though the cellulose is in solution and the spinning temperature is only moderately elevated. The term xe2x80x9ccontinuously drawnxe2x80x9d refers to the present commercial process for manufacture of lyocell fibers where they are mechanically pulled, first through an air gap to cause elongation and molecular orientation then through the regeneration bath.
The processes involve dissolving a cellulosic raw material in an amine oxide, preferably N-methylmorpholine-N-oxide (NMMO) with some water present. This dope, or cellulose solution in NMMO, can be made by known technology; e.g., as is discussed in any of the McCorsley or Franks et al. patents aforenoted. In the present process, the dope is then transferred at somewhat elevated temperature to the spinning apparatus by a pump or extruder at about 90xc2x0 C. to 130xc2x0 C. Ultimately the dope is directed through a multiplicity of small orifices into air. In the case of melt blowing, the extruded threads of cellulose dope are picked up by a turbulent gas stream flowing in a generally parallel direction to the path of the filaments. As the cellulose solution is ejected through the orifices the liquid strands or latent filaments are drawn (or significantly decreased in diameter and increased in length) during their continued trajectory after leaving the orifices. The turbulence induces a natural crimp and some variability in ultimate fiber diameter both between fibers and along the length of individual fibers. This is in marked contrast to continuously drawn fibers where diameters are uniform and crimp is lacking or must be introduced as a post spinning process. The crimp is irregular and will have a peak to peak amplitude greater than about one fiber diameter and a period greater than about five fiber diameters.
Spunbonding can be regarded as a species of meltblowing in that the fibers are picked up and drawn in an airstream without being mechanically pulled. In the context of the present invention meltblowing and spunbonding should be regarded as functional equivalents.
Where the fibers are produced by centrifugal spinning, the dope strands are expelled through small orifices into air and are drawn by the inertia imparted by the spinning head. The filaments are then directed into a regenerating solution or a regenerating solution is sprayed onto the filaments. Regenerating solutions are nonsolvents such as water, lower aliphatic alcohols, or mixtures of these. The NMMO used as the solvent can then be recovered from the regenerating bath for reuse.
Turbulence and oscillation in the air around the latent fiber strands is believed to be responsible for their unique geometry when made either by the melt blowing or centrifugal spinning process.
Filaments having an average size as low as 0.1 denier or even less can be readily formed. Denier can be controlled by a number of factors including but not limited to orifice diameter, gas stream speed, spinning head speed, and dope viscosity. Dope viscosity is, in turn, largely a factor of cellulose D.P. and concentration. Fiber length can be similarly controlled by design and velocity of the air stream surrounding the extrusion orifices. Continuous fibers or relatively short staple fibers can be produced depending on spinning conditions. Equipment can be readily modified to form individual fibers or to lay them into a mat of nonwoven cellulosic fabric. In the latter case the mat may be formed and become self bonded prior to regeneration of the cellulose. The fibers are then recovered from the regenerating medium, further washed, bleached if necessary, dried, and handled conventionally from that point in the process.
Gloss or luster of the fibers is considerably lower than continuously drawn lyocell fiber lacking a delusterant so they do not have a xe2x80x9cplasticxe2x80x9d appearance. This is believed to be due to their unique xe2x80x9cpebbledxe2x80x9d surface apparent in high magnification micrographs.
By properly controlling spinning conditions the fibers can be formed with variable cross sectional shape and a relatively narrow distribution of fiber diameters. Some variation in diameter and cross sectional configuration will typically occur along the length of individual fibers and between fibers. The fibers are unique for regenerated cellulose and similar in morphology to many natural fibers.
Fibers produced by either the melt blowing or centrifugal spinning processes possess a natural crimp quite unlike that imparted by a stuffer box. Crimp imparted by a stuffer box is relatively regular, has a relatively low amplitude usually less than one fiber diameter, and short peak-to-peak period normally not more than two or three fiber diameters. That of the present fibers has an irregular amplitude greater than one fiber diameter, usually much greater, and an irregular period exceeding about five fiber diameters, a characteristic of fibers having a curly or wavy appearance.
Quite unexpectedly, the fibers of the present invention appear to be highly resistant to fibrillation under conditions of wet abrasion. This is a major advantage in that no post spinning processing is required, such as crosslinking or enzymatic treatment.
Properties of the fibers of the present invention are well matched for carding and spinning in conventional textile manufacturing processes. The fibers, while having many of the attributes of natural fibers, can be produced in microdenier diameters unavailable in nature. It is possible to directly produce self bonded webs or tightly wound multi-ply yarns.
A particular advantage of the present invention is the ability to form blends of cellulose with what might otherwise be considered as incompatible polymeric materials. The amine oxides are extremely powerful solvents and can dissolve many other polymers beside cellulose. It is thus possible to form blends of cellulose with materials such as lignin, nylons, polyethylene oxides, polypropylene oxides, poly(acrylonitrile), poly(vinylpyrrolidone), poly(acrylic acid), starches, poly(vinyl alcohol), polyesters, polyketones, casein, cellulose acetate, amylose, amylopectins, cationic starches, and many others. Each of these materials in homogeneous blends with cellulose can produce fibers having new and unique properties.
It is an object of the present invention to provide a method of forming low denier regenerated cellulose fibers or cellulose blend fibers from solution in an amine oxide-water medium by processes analogous to melt blowing, spunbonding, or centrifugal spinning.
It is a further object to provide low denier cellulose fibers having advantageous geometry and surface characteristics for forming into yarns.
It is still another object to provide fibers having natural crimp and low luster.
It is an additional object to provide a lyocell fiber resistant to fibrillation under conditions of wet abrasion.
It is also an object to provide regenerated cellulose fibers having many properties similar or superior to natural fibers.
It is yet another object to provide a method of forming fibers of the above types by a process in which all production chemicals can be readily recovered and reused.
It is another object to provide self bonded nonwoven lyocell fabrics.